Now, in the market of optical recording medium there have been distributed rewritable optical recording media and so-called write-once optical recording media which are not rewritable. A rewritable optical recording medium, as meant by the word “rewritable,” allows data to be written therein again and again, so that it is possible for the same optical recording medium to be used repeatedly in recording only required data. On the other hand, a write-once optical recording medium is not rewritable and thus characterized by a feature that “data will never be altered,” thereby making itself useful in data distribution, storage, or backup.
In recent years there has been a demand for an optical recording medium which is capable of performing a stable high-density and high-speed recording when being used as multimedia for recording and editing music or moving images. As one of concrete measures, an attention is being given to an optical recording medium capable of performing recording/playback by means of a laser beam having a blue wavelength.
Conventionally, as a recording structure of a write-once optical recording medium, it has been suggested that organic dyes be applied to a substrate. However, such a recording structure has been found insufficient in its recording sensitivity when performing a high speed recording. On the other hand, if the wavelength of a laser beam is made short in order to increase a recording density, there will be a problem that it is difficult to synthesize dyes which can be used with laser beams having wavelengths equal to or shorter than that of a blue light.
Although there have been several suggestions (for example, Japanese Patent Publication No. Hei 4-838, etc.) that recording layer can be formed by an inorganic material, none of conventional structures is suitable for high density or high speed recording, their storage reliability of recorded state is inadequate, and their playback durability is insufficient.
For this reason, among various high density optical recording media using laser beams having wavelengths equal to or shorter than that of a blue light, what has been suggested as an optical recording medium capable of “high speed recording” is only a rewritable (RW) optical recording medium formed by using a phase-change material.
As one of important items required by a recording medium for use as a multimedia-compatible medium in recent years, there has been “stable recording at a variable speed” apart from “high-density and high-speed recording”. For example, when recording music or moving images, it is absolutely necessary that the recording be performed along a real time. On the other hand, there has been a demand for using a high speed in performing a recording for editing the above information or making a copy for backing up a file which has already been completed (because the capacity of the file is large). For this reason, in recent years, “using the same recording medium to flexibly satisfy various uses from a low speed to a high speed” has become a significant task for an optical recording medium which can be used as a multimedia-compatible medium.
Further, although an optical recording medium, which has been treated such that its recording density has been improved in preference to other parameters, can be used to perform a recording by using CLV (Constant Linear Velocity) method, since a considerable amount of time is needed in controlling the rotational speed of the optical recording medium, such an optical recording medium is not suitable for use as a recording medium which is required to perform a high speed access and a high speed transfer. On the other hand, by making it possible to perform a stable recording at a variable speed, it is also possible to adopt ZCAV (Zoned Constant Angular Velocity) method which makes the rotational speed constant and ensures the same high density as in the CLV method.
As mentioned above, among high density optical recording media using laser beams each having a wavelength equal to or lower than the wavelength of a blue light, an optical recording medium which is capable of performing a high speed recording and has already been suggested is only a rewritable recording medium containing a phase-change material.
However, a rewritable optical recording medium, in an aspect of “recording at a variable speed” as mentioned above, has a problem that its flexibility is low. This is because an optical recording medium utilizing a phase-change material is realized by distinguishing two different states, one of which is an amorphous state formed by quenching and the other is a crystalline state formed by slow cooling (kept at a temperature equal to or higher than a crystallizing temperature for a predetermined time). Accordingly, in view of a heat interference, and since a time control of laser pulses is complex, it is usually necessary to employ a recording strategy having laser powers of three or more different levels, i.e., a recording strategy having three or more powers, as well as to perform a time control for each laser pulse.
Here, so-called recording strategy means a power control pattern of a laser beam for recording. Generally, it is not that a laser beam for recording (particularly when recording is performed on an optical recording medium formed by using a phase-change material) is used to continuously perform an irradiation corresponding to the length of recording mark. Rather, it is often that an irradiation is performed by using a laser beam as a pulse train consisting of a plurality of pulses and the width of each pulse in the pulse train is strictly controlled so as to control the shape of recording mark, as described in Japanese Patent Laid-Open Publication No. Hei 9-7176. A detailed arrangement of pulse dividing at this time is generally referred to as recording strategy.
Since it is necessary to strictly control various factors such as a cooling speed related to time, a rewritable optical recording medium is difficult in adjusting its recording strategy when a recording transfer rate has been changed, and its flexibility with respect to a variable speed recording is extremely low.
Moreover, a rewritable optical recording medium is weak in preventing a data alteration which is possibly caused by an intentional action or a negligence. Therefore, with regard to a high-density optical recording medium which can be used as multimedia and can often be used to handle files such as video and music having a high value as works, it has been desired to produce a rewritable optical recording medium capable of performing a recording with a high density and at a variable speed, i.e., from a low speed to a high speed.